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RENEWABLE ENERGY REPORT LIBRARY LOG HOMES GLACIER PROJECT REPORT STATF nnnilMFNTS COLLECTiOr^ .MIL 2 0 1992 MONTANA STATE LIBRARY 1515 E- §th AVE. HELENA, MONTANA 59620 ^ ? TV. •V-4 ll INl i Prepared for Montana Department ofNatural Resources and Consenration , /v.‘> .V* '1 i . .'*t-%.'^v'':V5”-i;,v'i-'i'’'- '‘/v'.'i'ns'."''V' ., "!V'";^''^€'î‘;''V^V^;i:4'^^^^ ,'; ;'..M-v;.:*;r-''y'^''îrw.,yv'> .,•'y",',,-ryi:y.y’'-• „'»"'(*:?' y; '«siliwB»i I# ' . I’/>•'•;•"«’., •?••,T"’V•••<??'•• ?/•'vy >'• m mm w fifeh,tj,âli!m M- m ?iifr*Tiy; WP II \'i .'-“S y 1 ;4J .... s». J5t? J!>H iM' S.'? *-5j;' .V;# iiSi'-i' .M..'t?5 GLACIER LOG HOMES REPORT PROJECT Prepared by Peter Koch, President Wood Science Laboratory, Inc. 942 Little Willow Creek Road MT Corvallis, 59828 Phone (406) 961-4131 August 1991 Prepared for Montana Department ofNatural Resources and Conservation 1520 East Sixth Avenue Helena, MT 59620-2301 Available from Montana State Library, 1515 East Sixth Avenue, Justice and State Library Building, Helena, MT 59620 Thisreportwasprepared underAgreementNo. RAE-86-1070, fundedbytheMontana DepartmentofNatural Resources andConservation (DNRC). Neitherthe Department noranyofitsemployeesmakesanywarranty, expressorimplied, orassumesanylegal liabilityorresponsibilityforthe accuracy, completeness, orusefulnessofanyinforma- tion apparatus, product, or process disclosed, or represents that its use would not infringe on privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, doesnotnecessarilyconstituteorimplyitsendorsement, recommendation, orfavoring by the Department ofNatural Resources and Conservation or any employee thereof. The useragrees to hold DNRC harmless against any direct or consequential damages claimed by the user oftheir parties arising from or related to use or interpretations of this report. CONTENTS Page ABSTRACT iv INTRODUCTION 1 PROJECT HISTORY 3 BACKGROUND 3 CHRONOLOGY OF EVENTS 5 SYSTEM OPERATION 9 CONTROL SYSTEM 17 ENERGY REQUIREMENT 18 OPERATIONAL EXPENSES AND COMPARISON WITH OTHER SYSTEMS 19 SUGGESTED IMPROVEMENTS 20 RELATED RESEARCH NEEDS 20 REFERENCES 21 CHRONOLOGICAL LOG OF COMMUNICATIONS IN THE DNRC FILES RELATED TO THE GLACIER LOG HOMES PROJECT 22 0 ABSTRACT On June 15, 1986, the Deportment of Natural Resources and Conservation entered into a grant agreementwith Glacier Log Homes, Inc. ofWhitefish, Montana. The original intent ofthe agreementwasto demonstrate the economic and technical viabilityofa dehumidificationdrykilnemploying anelectricheatpump(compressor- refiigeration unit) to dry 6''-by-6" ond d^-by-S” western softwood green timbers to a moisture content of 17 percent of oven-dry weight within 10 days. Toward this end, DNRC ultimately paid Glacier Log Homes (Grantee) $108,769.61 of a projected $117,OCX)total grant amount; the Grantee invested approximately $50,000 ($31,800 documented, thebalanceestimated) ofGlacierLog Homes'sfundsto constructthe dry kiln, fuel delivery system, and controls. As actually built, the kiln system differs significantly from the originally intended design and is unable to achieve the target drying time. The system consists of a wood-fired, forced-air, low-temperature dryer with a holding capacity of40,000 boardfeet (BF) oftimber. Itisheatedthrough an air-to-airheatexchangerbyflue gas from a modified Dutch oven fueled by headrig sawdust; kiln humidity is not controlled. Ittakes about 30days to dry (to 17 percentmoisture content andwith kiln temperatures not exceeding 110°F) 6"-by-6" and 6"-by-8" timbers cut ft'om partially air-dried logs oflodgepole pine, white pine, spruce, fir, and larch. Connected electric motors total 62 horsepower (HP). If the kiln is continuously operated, kilning costs (including fuel, labor, maintenance, utilities, depreciation, overhead, and return on capital) likelywill total approximately $85/thousandboardfeet (MBF) dried. Because of log shortages, however, the kiln probably will operate only intermittently in the immediate future. 0 INTRODUCTION — Montana'ssizablelog andtimberhomesindus- months at a dry-bulb temperature of 70°F and try continues to grow. Most of the homes sold are relative humidity of82 percent, they ultimatelywill erected outofstate, andmanyhomes are exported to reach equilibrium at about 17 percent moisture Japan and Korea. During 1988, Montana log home content. manufacturers had aggregate gross annual sales of Toplacethisproblem inbetterperspective, the about $30 million (Keegan et al. 1991). The energy- — stemwood of live trees standing in the forest or conserving, thermal-mass effect for heavy log and — freshly harvested contains about half water. For timberwalls in residential construction is ofparticu- larinteresttoenergy-consciousconsumers(LogHomes example, if a fres—h green log we—ighs 1(X) pounds, Council n.d.). halfofthatweight or50pounds typicallyiswater (Koch 1987). To use wood technologists' terminol- Becauseofthenatureofloghomeconstruction, ogy, such wood is said to have 100percent moisture logs should be dried to equilibrium with the atmo- content; that is, the water in the wood weighs 100 spheric conditions under which the log structure percent of the weight of the oven-dry wood. ultimatelywillserve;thisequilibrationreducesshrink- age in wall height after erection to an acceptable When wood is being dried, it doesnotbeginto level. The atmospheric condition may vary from 10 shrink until it reaches about 28 percent moisture percent relative humidity (e.g., eastern Montana or content. Asthesoftwoodstypicallyusedinloghome Arizona during the summer) to 90 percent (e.g., constructiondryfrom 28percentmoisturecontentto Louisiana, Mississippi, or other warm, humid mari- 0 percent moisture content (oven-dry), they shrink time climate areas). The corresponding equilibrium radiallyto the log axis about4.3 percent. A log wall moisture content (EMC) of the wood comprising the 100 inches high built from green logs will therefore logs in the structure may range from 3 to 20 percent shrinkfrom its 100-inchheightto a 95.7-inchheight of the wood's oven-dry weight. In North America, ifthelogs are completelyoven-dried. Thisshrinkage average EMC in unheated wood structures generally is approximately linear with moisture loss between is considered to be about 12 percent of the wood's 28 and 0 percent moisture content. Thus, this 100- oven-dryweight. This average ultimately is reached, inch-high log wall will begin to shrink when log for example, at a dry-bulb temperature of 70°F and moisturecontentreoches28percent,andbythetime relative humidity of 65 percent. In homes, interior moisturecontenthasdroppedto 14percent, thewall heated walls commonly range from 3 to 10 percent height will have been reduced to 97.8 inches. At 17 moisture content, and exterior walls from 12 to 20 percent moisture content, the wall height will not percent. havebeenreducedquitesomuch—probablytoabout 98.3 inches. Tangential (circumferential) shrinkage For log home manufacturers, these low mois- in logs is about 6.7 percent from green to oven-dry; ture contents are difficult to reach, and most manu- this is the shrinkage that opens longitudinal checks facturerslikelyshiplogsandheavytimbercomprising (cracks) in log surfaces. log home kits at a moisture content of about 17 percent of oven-dry weight. In Montana, since out- The foregoing discussion explains why win- side air temperatures and humidities vary widely dowanddoorframesandinteriorwallfacings(which among seasons, generalization about drying rates is do not shrink appreciably in heightwith changesin not possible. As an example, however, if logs are — moisture content) arebestinstalled afterlog shrink- equilibrated for a sufficient time a matter of age has taken place. Thus, log and timber moisture contentmustbereducedtoanacceptablelevelbefore air, and heat losses from kilns are substantial. Also, house kits are shipped to consumers. kiln heat losses positively correlate with residence timeinthekiln;thatis,heatlossesaregreaterforlong Nowconsiderhowmuchwatermustbe evapo- kiln schedules than short. A rated. cubic foot of lodgepole pine logs (oven-dry — wood only no water) of the diameter commonly In conventional and high-temperature kilns used for house logs weighs about 25 pounds. This for large-scale drying of southern pine lumber, 3 same cubic footfrom freshly cut, live lodgepole pine million Btu/MBF of green lumber dried to market trees also contains about 25 pounds of water. (A moisture content is a commonly used estimate of family-size log home might be comprised of 800 total energy requirement (Wengert and Beckwith — cubic feet oflogs and timbers.) To dry one cubic foot 1989). Ifkiln schedulesare long protracted usually — from the green condition (100 percentmoisture con- the case when drying timbers energy consumption tent) to 17 percent moisture content requires evapo- can be much greater. ration of 20.75 pounds of water (83/100 x 25). Generalizationabouttheheatrequirementofa Since the relationship of energy to dry logs is kiln drying lodgepole pine, white pine, spruce, fir, more or less linear to the amount of water to be and larch house logs and heavy timbers is difficult. evaporated, log home builders understandably pre- This difficulty arises in spite of fixing the ending fertoacquirestanding deadtimberthathasair-dried moisture content at 17 percent, because the starting on the stump and lost a significant amount of its moisturecontentmayvaryfrom 100percentofoven- water. Data from Lowery and Hearst (1987) suggest dry weight for fresh green wood to about 24 percent that lodgepole pine killedby bark beetles or fire and forwood air-driedforlong periods. Also, theambient left on the stump for a year or more will attain a wood and airtemperaturesmayvaryfrom below0°F moisturecontentbelowthefibersaturationpoint(28 in winter to perhaps 80®F in summer. Thermal heat percent of oven-dry weight). If sawed into lumber, supply to a kiln must meet maximum demand, the wood will yield 2”-by-4" studs with a moisture however. If green log and timber input is assumed, content of abo—ut 24 percent of oven-dry weight. and the overall kiln thermal efficiency is 40 percent, During drying ^whether on the stump, in an air- one cubic foot of logs or timbers will require about — drying yard, or in a kiln logs or heavy timbers will 50,318 Btu ofheat energy to dry this cubic foot to 17 check. Ifnot too severe, however (e.g., less than 1/4- percentmoisturecontent. Fanenergytocirculatekiln inchwide andlessthan one full turn ofspiral grain), air, provide combustion air, and furnish furnace this defect usually is acceptable to the log home draft, along with energy to operate fuel conveyors, builder. Less acceptable to some is the severe blue must be provided in addition to the thermol energy. stain accompanying a beetle attack and extended drying on the stump. A pound of wood (oven-dry) contains about 8,6(X) Btu. When heat exchangers are used in con- In any event, forone cubicfootoflogsto reach junction with wood-burning furnaces, the combined 17percentmoisturecontentrequirestheevaporation efficiency of combustion and heat transfer may fall of 1.75 to 20.75 pounds of water, depending on within a range of 60 percent with green fuel to 70 whether the logs have been dried on the stump (as percent with dry fuel. Ifwood fuel hasbeen partially dead trees) or freshly cut from live trees. The heat dried, combined combustion and heat transfer effi- energy required to evaporate a pound of water at ciency is estimated at 65 percent. atmospheric pressure is at least 970 Btu (British thermal units). Therefore, at 100 percent efficiency, Providing the 50,318 Btu of heat energy re- theoretical minimum heatrequiredto evaporate the quired to dry one cubic foot of green wood to 17 desired amount ofwater from one cubic foot oflogs percent moisture content, therefore, requires about rangesfrom about 1,698to20,127Btu,dependingon 9.0 pounds (50,318/8,600/0.65) of woody, air-dried the initial moisture content of the logs. In actual fuel (oven-dryweightbasis). Forthoroughlyair-dried practice,however,theamountissignificantlygreater timbers (24 percent moisture content) dried to 17 because logs are cool, fan energy is needed to move percent moisture content, fuel demand will be 0 reduced significantly. If kiln schedules ore greatly inventories and because significant land acreage is — extended frequently the case when drying timbers required forthis type ofdrying. To reduce log inven- — that permit minimal checking fuel consumption tories and log-yard acreage, and to control moisture may be increased significantly. contentin logsand timbersshipped, log home builders are interested in the kiln-drying process. This The time needed to air-dry logs and timbers is interest in kiln-drying and the energy consumption measured in months. As a result, air-drying can be of kilns for logs and timbers provided the driving expensive because of the cost of carrying large log force behind the Glacier Log Homes project. PROJECT HISTORY BACKGROUND is condensed, it releases the heat of condensa- — tion identical to the amount of heat that was Areviewofthecorrespondenceanddocuments required to evaporate the waterfrom a dish. The (February 1986 through March 1991) related to the compressorthen pumps thisheattothe hotcoils. Glacier Log Homes project suggests that objectives (Inanairconditionerorrefrigerator, thehotcoils werenotclearatthe outset. It seemsto havebeen an are outside.) The dried air from the kiln is then early intent of Glacier Log Homes, Inc. (Grantee) to passedoverthesehotcoils, pickingupthisenergy builda dehumidificationkiln.Totheextenttowhich ofcondensation. In heating the air, its humidity the ultimate system departed from this intent, a is lowered. Ifthe temperature difference between discussion ofthe principle ofoperation ofdehumidi- the hot coils and cold coils is 50 degrees F., then thehumidityofthe exiting airwillbe about 15% fication dryers is useful. Wengert et al. (1988) de- RH. The air then enters the kiln, passes through scribe the dehumidification kiln as follows: the stack oflumber, evaporating more moisture, and eventually back to the dehumidifier. The dehumidification drykiln offers a methodof drying wood (i.e., controlling the temperature Experts advise that dehumidifying dryers are and humidity) without using a large boiler for best used in applications where: heatandhumidificationandwithoutusingvents for lowering the relative humidity. Instead of • small-scale drying is contemplated (chambers steamandventing,thedehumidifieruseselectric- typically hold 30 MBF ofwood) itytocool the airandtherebycondense moisture • fuels alternate to electricity are not available from the air. In addition, because there isalarge amountofheatreleasedwhenwateriscondensed, • electricity costs are low [less than $0.07/kilowatt hour (kWh)] thedehumidifierrecyclesthisheatintothekilnto evaporatemoisture.Theequipmentusedisaheat pump (also called a compressor or refrigeration With dehumidification drying oftimbers, tem- unit). The heatpumpoperates on the same prin- peratureswouldnotexceed 150°F(onlycertaingrades cipleasawindowairconditioneroracompressor ofrefrigerant will permit such a high temperature). inarefrigerator.Figure 1 [seepage4]showsdamp In very permeable woods such as southern pine, airbeingwithdrawnfromthekiln. (Notalltheair drying 6"-by-6" timbers to 17 percent moisture con- — iswithdrawn onlyasmallportionofit.)Thisair tent should take about 15 days. With impermeable is passed through the cold coils (also called the softwood such as lodgepole pine heartwood, drying evaporator coils) of the compressor. The humid time likely would take 30 days, and temperatures air cools and as it cools below the dew point, would be lower—perhaps less than 120°F. The com- moisture is condensed. The colder the coils, the more thatwill becondensed. When thismoisture pressor on this type of dryer of impermeable wood 0 Figure 1. Schematic diagram ofa dehumidiftcation dryer. Drawing from Wengert et al. (1988). — — usually is sizedwith one HP perMBF holding capac- This condensed water usually slightly acidic is ity in the kiln. Capital cost for compressors is about sewered, sometimes being chemically neutralized $1.50/BF of kiln holding capacity of impermeable first with soda ash. wood. Total cost of the kiln and compressor for 40 MBFholdingcapacityofrelativelyimpermeablewood Atleast5(X)suchkilnsoperateinNorthAmerica. (such as lodgepole pine) is about $120,000. Three well-known manufacturers are: T0supplytheauxiliaryheatsometimesneeded • Uraken, St. Eustache, Quebec, Canada (see Figure 1), and to supply high humidity for con- • Dry Line System, Mississauga, Ontario, Canada ditioning timbers at the end of the drying cycle, a • Nyle Corporation, Bangor, Maine, United States small electricboiler commonly is used (the compressor is shut off during this conditioning cycle). Alter- The following discussion will focus on events in- natively, the small auxiliary boiler is fired with volved with the Glacier Log Homes project and ab- natural gas or propane. stracts related correspondence. (A chronological list of communications related to the project begins on As previouslynoted, the wateroutputperMBF — page 22.)^ of wood dried is considerable about 250 gallons. ^ These documents and othersmentioned throughoutthisreportthatrelatetothe GladerLog Homes projectareonfile atthe Department ofNatural Resources and Conservation in Helena. ©